FIG. 1 shows a trench transistor of the type referred to above. A trench transistor 1 has a first electrode 2, a second electrode 3 and also a semiconductor body 4 arranged between the first and second electrodes 2, 3. A plurality of transistor cells 5 are formed in the semiconductor body 4. Each transistor cell has a source region 6, a body region 7, a drift region 8 and also a gate electrode 9. The gate electrode 9 is provided within a trench 10 extending from the top side of the semiconductor body 4 in the vertical direction into the semiconductor body, the gate electrode 9 being insulated from the semiconductor body 4 by an insulation layer 11. Contact holes 12 serving for making contact with the source and body regions 6, 7 are furthermore provided. The contact holes 12 are completely filled by the first electrode 2, which is composed of a metallic material. The bottom 13 of each contact hole 12 adjoins the drift region 8, whereby the bottoms 13 form Schottky contacts between the first electrode 2 and the drift region 8. Furthermore, a portion 14 of the side walls of the contact holes 12 likewise constitutes a Schottky contact. A highly doped connection zone 15 adjoining the second electrode 3 is formed in the semiconductor body 4. The source regions 6 and also the gate electrodes 9 are electrically insulated from the first electrode 2 toward the top by means of an insulation layer or passivation layer 16.
The trench transistor 1 shown in FIG. 1 is used in particular for switching inductive loads, for example motors. As long as the trench transistor 1 is operated in the forward direction, that is to say as long as a positive drain/source voltage is present, the Schottky contacts in between the first electrode 2 and the drift region 8 effect blocking. However, if a negative drain/source voltage is present, then the Schottky contacts conduct and thus function as freewheeling elements. A negative drain/source voltage may be brought about for example during the switching of inductive loads by the trench transistor 1 on account of a voltage which is induced by the inductive load after the trench transistor 1 has been turned off.
As can be gathered from FIG. 1, the Schottky contacts are connected in parallel with body diodes formed by the pn junctions between the body regions 7 and the drift region 8 and also the short circuit between the body regions 7 and the source zones 6. The forward voltages of the Schottky diodes are lower than the forward voltages of the body diodes, so that the Schottky diodes always turn on before the body diodes.
The advantage of the Schottky diodes is that in contrast to the body diodes, when the trench transistor 1 is operated in the forward direction, the transistor cells 5 do not store any charge carriers which have to be depleted from the transistor cells 5 again when the trench transistor 1 undergoes transition to the off state. Consequently, the Schottky diodes contribute to reducing switching losses during the switching of inductive loads.
What is disadvantageous about the trench transistor 1 described in FIG. 1 is that excessive increases in the electric field strength can readily occur at corners 17 (corners of the contact holes 12). However, high electric field strengths entail high leakage currents, which is undesirable. The leakage currents are additionally intensified as reverse voltages lying in the region of the breakdown voltage of the trench transistor 1.